The cerebral cortex of the rat contains an anatomically distinct area consisting of cylinders of neurons, or barrels, in lamina IV of somatic sensory area I, SI. All neurons of a particular barrel are excited by only one contralateral mystacial vibrissa. As a follow-up to our light microscopic study of the projections from the ventrobasal complex to the barrels, we shall determine the corticocortical connections to and from the barrel field. Aspiration lesions will be made in this region and other ipsilateral and contralateral cortical areas and then cortical axonal degeneration patterns will be studied by means of the Fink-Heimer technique. Fine structure of normal cells of the barrels and the mode of termination of degenerating thalamocortical afferents following large ventrobasal complex lesions will be examined under the electron microscope.